"In Euphorbia, the small structure that appears to be a single flower  technically known as a cyathium  is a lobed, cup-shaped involucre with glands at the bases of the segments, containing one or more staminate flowers (each consisting of a single stamen only) surrounding a solitary pistillate flower (a stalked ovary topped by 3 styles).

"In some species the involucre also bears appendages that simulate petals. (The well-known Poinsettia is a Euphorbia in which brightly colored leaves beneath the inflorescence give the impression of a large-petaled flower.)"  Wildflowers of the Southern Mountains, by Richard M. Smith